Absorbent nonwoven pulp fiber webs have long been used as practical and convenient disposable hand towels or wipes. These nonwoven webs are typically manufactured in conventional high speed papermaking processes having additional post-treatment steps designed to increase the absorbency of the paper sheet. Exemplary post-treatment steps include creping, aperturing, and embossing. These post-treatment steps as well as certain additives (e.g., debonding agents) generally appear to enhance absorbency by loosening the compact fiber network found in most types of nonwoven pulp fiber webs, especially those webs made from low-average fiber length pulp such as, for example, secondary (i.e., recycled) fiber pulp.
Some highly absorbent single ply and multiple-ply absorbent hand towels or wipes are made using the conventional methods described above. Those materials, which may be capable of absorbing up to about 5 times their weight of water or aqueous liquid, are typically made from high-average fiber length virgin softwood pulp. Low-average fiber length pulps typically do not yield highly absorbent hand towels or wipes
While a loosened network of pulp fibers is generally associated with good absorbency in nonwoven pulp fiber webs, such a loose fiber network may reduce the rate which the nonwoven pulp fiber web absorbs and/or wicks liquids.
Water jet entanglement has been disclosed as having a positive effect on the absorbency of a nonwoven wood pulp fiber web. For example, Canadian Patent No. 841,398 to Shambelan discloses that high pressure jet streams of water may be used to produce a paper sheet having a highly entangled fiber structure with greater toughness, flexibility, and extensibility, abrasion resistance, and absorbency than the untreated starting paper. The fabrics are prepared by treating a paper sheet with jet streams of water until a stream energy of 0.05 to 2.0 horsepower-hours per pound of product has been applied in order to create a highly entangled fiber structure characterized by a considerable proportion of fiber segments aligned transversely to the plane of the fabric. According to Shambelan, these fabrics are characterized by a density of less than 0.3 grams/cm.sup.3, a strip tensile strength of at least 0.7 pounds/inch per yd.sup.2, and an elongation-at-break of at least 10% in all directions. It is disclosed that the entangled fiber structure may be formed from any fibers previously used in papermaking as well as blends of staple length fibers and wood pulp fibers.
A paper entitled "Aspects of Jetlace Technology as Applied to Wet-Laid Non-Wovens" by Audre Vuillaume and presented at the Nonwovens in Medical & Healthcare Applications Conference (November 1987) teaches that in order to successfully entangle short fibers like wood pulp fibers it is necessary to add long fibers (e.g., staple length fibers) to create a coherent web structure. The addition of 25 to 30% long fiber is recommended. The paper also recommends utilizing jets of water at less than conventional pressures to entangle the fibers because high-pressure jets of water would destroy or damage the web and/or cause unacceptable fiber loss.
An exemplary wet-laid nonwoven fibrous web which is hydraulically entangled at reduced entangling energies is disclosed in U.S. Pat. No. 4,755,421 to Manning, et al. That patent describes a wet-wipe formed from a wet-laid web containing wood pulp fibers and at least 5 percent, by weight, staple length regenerated cellulose fibers. The web is treated with jet streams of water until a stream energy of 0.07 to 0.09 horsepower-hours per pound of product is applied. The treated web is disclosed as having high wet tensile strength when packed in a preservative liquid yet is able to break up under mild agitation in a wet environment. According to Manning, et al., the breakup time and wet tensile strength is proportional to the entangling energy. That is, as entangling energy is reduced, the wet tensile strength and the break-up time are reduced.
While these references are of interest to those practicing water-jet entanglement of fibrous materials, they do not address the need for a water jet treatment which opens up or loosens a compact network of pulp fibers to produce a highly absorbent nonwoven web which may be used as a disposable hand towel or wipe or as a fluid distribution material in a personal care product. There is still a need for an inexpensive nonwoven pulp fiber web which is able to quickly absorb several times its weight in water or aqueous liquid. There is also a need for a nonwoven pulp fiber web which contains a substantial proportion of low-average fiber length pulp and which is able to quickly absorb several times its weight in water or aqueous liquid. There is also a need for a practical method of making a highly absorbent pulp fiber web. This need also extends to a method of making such a web which contains a substantial proportion of low-average fiber length pulp. Meeting this need is important since it is both economically and environmentally desirable to substitute low-average fiber length secondary (i.e., recycled) fiber pulp for high-quality virgin wood fiber pulp still provide a highly absorbent nonwoven pulp fiber web.